Small change

August 25, 2008

NIU physicists are shedding new light on nanoscale superconducting materials, demonstrating how their properties change when the materials are reduced to a single dimension.

Recent NIU graduate Jiong Hua, now a joint NIU/Argonne National Laboratory postdoctoral research associate, is the lead author on the latest research, published in the Aug. 15 issue of the prestigious journal, Physical Review Letters.

Other authors on the paper include NIU Physics Professor Zhili Xiao and his Ph.D. students Suhong Yu and Umeshkumar Patel, as well as Argonne scientists.

“It is extremely difficult to publish an article in Physical Review Letters, which has strict criteria not only for the importance and validation but also the broad interest of the results,” Xiao said.

“This research article is based on Jiong’s dissertation work and was submitted while he was still a student here,” Xiao added. “I am very proud of him. Before Jiong joined the physics department, he had been specializing in electrical engineering at NIU. However, he became one of the best physics graduate students and was the first recipient of the NIU/Argonne Distinguished Graduate Fellowship.”

The behavior of materials at increasingly smaller length scales is a dominant theme in modern physical science. The last decade has seen the discovery of qualitatively new behaviors in metals, in semiconductors and in magnets as scientists’ ability to fabricate and probe samples at small-length scales evolves.

The team of researchers studied how the properties of a superconducting material change at the nanoscale. When a magnetic field is applied to isotropic materials at normal-size scales, their properties remain unchanged with changing field direction. But if a material’s dimensions are reduced, for example to an incredibly thin film, its properties become anisotropic, which means they depend on the direction of an applied magnetic field.

Because thin film is easy to fabricate, theories on anisotropic properties of a thin film have been tested intensively. However, very little research has been done on the properties of materials with two reduced dimensions, such as nanostrips and nanowires, because they are extremely difficult to create.

The world-class nanofabrication facilities at Argonne’s Center for Nanoscale Materials enabled the scientists to study the physical properties in strips and wires. Through collaborating with Argonne scientists, the NIU research team developed a new approach to achieve high quality superconducting nanostrips by utilizing the advantages of both electron-beam lithography and focused-ion-beam milling techniques.

“Our team studied samples of the transition metal superconductor niobium,” Hua says. “We found that, with a one-dimensional strip, the angular dependence of the resistance and the transition from normal state to superconducting state differs from that of a three-dimensional bulk sample and a two-dimensional thin film in the presence of a magnetic field.”

The team interpreted this confinement effect as a size-dependent exclusion of the magnetic field by a superconductor at the nanoscale.

“The research is important in the field of nanotechnology,” Xiao said.

“Small superconducting wires and strips are highly desirable as potential interconnects in future electronic nanodevices since they circumvent the damaging heat produced by energy dissipation in a normal nanoconductor,” he added. “Information on their anisotropic properties in the presence of a magnetic field is crucial in designing the devices because current-flowing electric circuits produce a magnetic field.”